High CPU/DB usage from ReplicationUserDevicesResyncRestServlet

[richvdh]

Sometimes our main process sees high CPU and DB usage due to thousands of requests to ReplicationUserDevicesResyncRestServlet:

We should find out what is calling this, and stop it doing so.

[richvdh]

(this is enough CPU and DB usage to stop synapse responding to other requests, which is problematic)

[squahtx]

May possibly have the same cause as #12890, where we saw a client endlessly retry POST /_matrix/client/v3/keys/query (which calls ReplicationUserDevicesResyncRestServlet) because it kept timing out. Each request went into a linearizer queue, behind previous requests, which guaranteed another timeout.

[richvdh]

Yes, that looks right. I'll close this in favour of #12890.